1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to an aligning method under electric field of ferroelectric liquid crystal and a liquid crystal display using the same for alignment restoration.
2. Description of the Related Art
A liquid crystal display applies electric field to the liquid crystal corresponding to the video signal, controls the arrangement state of the liquid crystal, and displays pictures by means of adjusting a light beam transmittance according to the video signal. The liquid crystal display includes a liquid crystal display panel having liquid crystal injected between two glass substrates. The liquid crystal display also includes a light source module or a back light unit for radiating light to the liquid crystal display panel. A structure, such as a frame and chassis, integrally fix the liquid crystal display panel and the light source module as one body. The liquid crystal display further includes a printed circuit board, hereinafter referred to as “PCB”, for applying a driving signal to the liquid crystal display panel.
The fabrication process of the liquid crystal display is divided into the steps of substrate cleaning, substrate patterning, substrate bonding, liquid crystal injecting, and drive circuit mounting processes. In the substrate cleaning process, a detergent is used to remove impurities from the surface of a substrate that will be used as the liquid crystal display panel. The substrate patterning process is divided into steps of upper glass substrate patterning and lower glass substrate patterning. On the upper glass substrate of the liquid crystal display panel, a color filter, a common electrode, and a black matrix are formed. On the lower glass substrate of liquid crystal display panel, data lines and gate lines are formed along with a thin film transistor at each intersection of the data and gate lines. Further, a pixel region is defined between the data and gate lines in which a pixel electrode is formed.
The substrate bonding/liquid crystal injecting process includes spreading an alignment film on the substrates of the liquid crystal display panel and rubbing the alignment film. The substrate bonding/liquid crystal injecting process also includes a process of adhering a polarizers on the upper and lower glass substrates. The polarizing directions of the polarizers perpendicularly cross each other. The substrate bonding/liquid crystal injecting process further includes the processes of bonding the upper glass substrate with the lower glass substrate using a sealant, injecting liquid crystal into an injection hole left in the sealant, and sealing the injection hole.
In the driving circuit mounting process, a tape carrier package TCP is connected to a pad part formed on the lower glass substrate. Integrated circuits, such as a gate drive integrated circuit and a data drive integrated circuit are mounted on the tape carrier package TCP. The drive integrated circuit can be directly mounted on the lower glass substrate by a chip on glass (COG) system instead of tape automated bonding system using the TCP described above.
When a liquid crystal display panel is manufactured by such fabrication processes, a module is constructed to have the liquid crystal display panel, the light source module, and the PCB as one body. In the process of constructing such a module, the PCB, the light source module, and the liquid crystal display panel are stacked in the cavity within a main frame, and a top case is constructed to attach to the main frame so that a side of the main frame and the edge of the liquid crystal display panel may be enclosed. In some cases, a bottom case located on the bottom side of the module encloses the bottom surface of the main frame and is also attached to the main frame. Herein, the input terminal of TCP is connected to the output pad of the PCB. The light source module includes a cold-cathode fluorescent lamp and a light guide panel, as well as a prism sheet and a diffusion plate, which are stacked between a light guide panel and a liquid crystal display panel.
The liquid crystal used in the liquid crystal display has a middle state between solid and liquid so as to have fluidity and elasticity at the same time. Up till now, most of the liquid crystal used has twisted nematic TN mode liquid crystal. The TN mode liquid crystal has defects in that its response speed is slow and its viewing angle is narrow. Ferroelectric liquid crystal FLC has the advantages of high response speed and a wide viewing angle.
Ferroelectric liquid crystal has a layer structure in which the electric and the magnetic properties are the same. Ferroelectric liquid crystal is driven in plane and rotates along a virtual cone in response to an electric field. The ferroelectric liquid crystal has a permanent polarization. In other words, spontaneous polarization occurs even though an external electric field is not applied, like an interaction between magnets. If an external electric field is applied, the ferroelectric liquid crystal rotates rapidly because the external electric field quickly counteracts the spontaneous polarization. So the response speed of the ferroelectric liquid crystal is hundred or thousand times faster than that of other modes of liquid crystal. Further, since the ferroelectric liquid crystal has an in-plane-switching property within itself, it can obtain the wide-viewing-angle without a special electrode structure or compensation film. The ferroelectric liquid crystal is classified into the V-Switching Mode and the Half V-Switching Mode according to the characteristic of reacting in response to the polarity of electric field.
FIG. 1 is a graph illustrating voltage vs. transmittance characteristic of ferroelectric liquid crystal of V-Switching mode. In the ferroelectric liquid crystal cell of V-Switching Mode, as the temperature goes down, a thermodynamical phase transition like isotropic→smectic A phase SA→smectic X phase Sm X*→crystal arises. Here, isotropic means the state that the liquid crystal molecules do not have a direction or location order. Smectic A phase means a state that the liquid crystal molecules are divided into virtual layers and are arranged perpendicularly on the virtual layers and have a symmetry in the above and below the virtual layer. The Smectic X phase means the middle state between the smectic A phase and the crystal phase. The ferroelectric liquid crystal cell of V-Switching Mode in which the liquid crystal molecules are transited to the smectic X phase, as shown in FIG. 1, improves the light beam transmittance of incidence light by means of changing the arrangement state, responding to the external voltage of positive polarity +V and the external voltage of negative polarity −V.
The V-Switching Mode has the advantages of a high-speed response and a wide-viewing-angle. However, the V-Switching Mode has the disadvantage of high power usage needed to drive the liquid crystal cell with a high data voltage because the spontaneous polarization value is large, and the capacitance of storage capacitor in order to maintain such a high data voltage is large. Accordingly, if the V-Switching Mode is used in a liquid crystal display, the aperture ratio of the panel is small since the electrode area of an auxiliary capacitor becomes large.
In contrast, the Half V-Switching Mode has the advantages of high-speed-response and wide-viewing-angle. FIG. 2 illustrates a phase transition process of ferroelectric liquid crystal of Half V-Switching mode. The Half V-Switching Mode can be used for displaying moving pictures with a good aperture ratio because the data voltages and capacitances are comparatively low. As shown in FIG. 2, in the Half V-Switching Mode ferroelectric liquid crystal, lowering the temperature below the transition temperature Tni causes a phase transition from isotropic to nematic phase N*. As the temperature goes below the transition temperature Tsn, a phase transition from the nematic phase N* to the smectic C phase Sm C* occurs. Below the transition temperature Tcs, a phase transition from the smectic C phase to crystal occurs. In other words, a thermodynamical phase transition of isotropic-nematic N*→smectic C phase Sm C*→crystal occurs.
FIG. 3 illustrates change of molecule arrangement depending on whether or not electric field alignment of ferroelectric liquid crystal of the Half V-Switching mode has occurred. In FIG. 3, the “{circle around (X)}” represents the electric field direction and the spontaneous polarization direction of the ferroelectric liquid crystal coinciding with the direction toward the drawings from an observer. Ferroelectric liquid crystal is injected into a liquid crystal cell at an incipient temperature such that the liquid crystal is isotropic without direction and location order. If this isotropic temperature is lowered, the ferroelectric liquid crystal becomes nematic phase N* arranged in parallel to the rubbing direction of the alignment film in the cell. While in the nematic phase N*, if the temperature is gradually lowered during the application of sufficient electric field inside liquid crystal cell, the ferroelectric liquid crystal of the nematic phase N* undergoes a phase transition to the smectic phase C* and the spontaneous polarization direction of the ferroelectric liquid crystal is arranged coincidentally with the direction of the electric field formed inside the cell.
As a result of aligning with the electric field, the liquid crystal cell has a spontaneous polarization direction that coincides with the direction of the electric field applied during the electric field alignment such that there is a uniform alignment condition in which layers of the two possible molecule arrangements are all formed in a single direction. On the other hand, without the electric field alignment process, layers of two molecule arrangements having different directions are formed after transiting from the nematic phase N* to the smectic C phase Sm*C. If a random bistable state in which the two molecule arrangements of the ferroelectric liquid crystal have different directions appears, it becomes difficult to control the ferroelectric liquid crystal. Thus, the ferroelectric liquid crystal cell of the Half V-Mode should be arranged to be in a monostable state by means of phase-transiting the ferroelectric liquid crystal from the nematic phase N* to the smectic C phase Sm C* while applying a low DC Voltage and lowering the temperature.
The electric field alignment of the ferroelectric liquid crystal cell of the Half V-Switching Mode is executed after the substrate bonding/liquid crystal injecting process described above. During electric field alignment, the data lines of the liquid crystal display panel are commonly connected to the shorting bar while the gate lines are commonly connected to another shorting bar. A scan voltage, which is more than threshold voltage of a TFT, is applied to the gate lines. A common voltage Vcom is applied to a common electrode of the upper glass substrate. At this time, a direct voltage is applied to the ferroelectric liquid crystal by the common voltage applied to the common electrode and the voltage applied to a pixel electrode through the data lines.
FIG. 4A and FIG. 4B are graphs illustrating the change of light beam transmittance according to the voltage in the ferroelectric liquid crystal cell of the Half V-Switching Mode. Referring to FIG. 4A, in the case of electric field alignment made by the voltage of negative polarity −V or the electric field of negative polarity, the ferroelectric liquid crystal cell of the Half V-Switching Mode makes the incident light beam transmit by means of converting the polarization direction of the incident light beam into 90° only when a positive voltage +V is applied, and makes the incident light beam nearly cut-off by maintaining the polarization direction of the incident light beam in case that the negative voltage −V is applied. The light beam transmittance increases in proportion to the positive electric field intensity and maintains a maximum value if the positive electric field intensity increases to more than the fixed threshold value. In contrast, the ferroelectric liquid crystal cell of the Half V-Switching Mode aligned under an electric field of positive polarity +V, as illustrated in FIG. 4B, makes the incident light beam transmit only in case that negative voltage −V is applied and makes the incident light beam nearly cut-off when a positive voltage +V is applied.
FIGS. 5A and 5B represent a change of a ferroelectric liquid crystal arrangement aligned by negative polarity electric field alignment for a ferroelectric liquid crystal cell of the Half V-Switching Mode and the change of the ferroelectric liquid crystal arrangement when the external electric field of the positive polarity and the negative polarity is applied. As shown in FIG. 5A, if the ferroelectric liquid crystal cell of the Half V-Switching Mode is aligned under electric field by an external electric field of the negative polarity E−, the spontaneous polarization direction Ps of the ferroelectric liquid crystal is aligned uniformly to the direction coinciding with the external electric field of the negative polarity E−. After the electric field is aligned like this, if the external electric field of the positive polarity E+ is applied to the ferroelectric liquid crystal cell of the Half V-Switching Mode, the arrangement of the ferroelectric liquid crystal is changed and the spontaneous polarization direction Ps coincides with the external electric field of the positive polarity E+ (FIG. 5B). At this moment, the polarization direction of the incident light beam from the lower plate of the liquid crystal display is changed to the polarization direction of the polarizer of the upper plate by the ferroelectric liquid crystal and the incident light beam transmits through the polarizer of the upper plate. If the external electric field of the negative polarity E− is applied or the external electric field is not applied to the ferroelectric liquid crystal cell of the Half V-Switching Mode, the arrangement of the ferroelectric liquid crystal maintains the incipient arrangement state and the incident light beam is cut off because the polarization direction is maintained.
The related art ferroelectric liquid crystal cell has a problem in that the original alignment is easily damaged by physical impact because the cell gap is narrow. After the process of the substrate bonding/the liquid crystal injecting, the original electric field alignment is likely to be damaged in the module construction process in which the physical impact is generated frequently. In order to restore the electric field alignment in such a ferroelectric liquid crystal display panel where the original alignment is damaged, the TCP has to be separated from the liquid crystal display panel and the electric field alignment voltage source has to be connected again to the signal wiring with common connections.